The invention relates to a method of constructing a floating structure comprising an elongate floating body and a deck structure connected to the upper end of the floating body, the method comprising the steps of:
providing the elongate body,
providing the deck structure, wherein the deck structure and the elongate body are each provided with coupling means for connecting the elongate body and the deck structure,
transporting the elongate body to its mooring site,
transporting the deck structure on a buoyancy member to the mooring site,
ballasting the elongate body such that it is in its vertically upright position and
connecting the deck structure to the upper part of the elongate body via the respective coupling means.
In the offshore oil industry, SPAR buoys are known for oil production and storage in deep water. The SPAR buoy comprises an elongate cylindrical lower part which may have a diameter of over 20 meters and a height or more than 100 meters. The deep draft cylindrical body is provided at its upper part with a super structure that may comprise oil or gas production equipment or a drilling rig. The cylindrical body is in its vertical position anchored to the sea bed by means of vertical tethers, and/or catenary or taut radial mooring lines. The subsea well head is connected to the floating body by a number of risers which transfer oil and/or gas to the upper deck structure. The main cylinder of the SPAR buoy may comprise storage and ballast tanks and may have a central well through which the risers extend to the production trees on the upper deck structures
It known to horizontally construct the main floating cylindrical body of the SPAR buoy, subsequently ballast it and tow it to its mooring site in a horizontal position. Thereafter the superstructure is placed on a barge and is also towed to the mooring site. By ballasting the floating body it is placed into its vertically upright position whereafter the superstructure is lifted as a whole or in parts from the barge by means of a derrick. This construction method is relatively costly and can only be carried out with large derricks, which may have to lift superstructures weighing 5000 tons or more.
Therefore it is an object of the present invention to provide a SPAR construction method which is relatively cheap and which can be carried out in a simple and reliable manner. Thereto the invention is characterized in that the relative position of the deck structure and the buoyancy member is maintained generally constant during alignment and connecting of the deck structure and the elongate body.
The invention is based on the insight that the buoyancy member of the deck structure should remain active during coupling of the SPAR body and the deck structure. When the SPAR body is erected by being ballasted with sea water, it can be brought to the required height so that it can pass underneath the floating super structure. Alternatively, the super structure may be deballasted in such a way that is high enough above water level to be able to pass over and be aligned with the upright floating SPAR body. After alignment of the deck structure and the vertical SPAR body, they are interconnected via the coupling means, wherein the deck structure may be lowered onto the SPAR body for instance by ballasting or winching via connecting cables, or wherein the SPAR body may be raised by deballasting, optionally in combination with winching along connecting cables. By making effective use of the buoyancy of the deck structure, large lifting derricks need not be applied during connection of the upright SPAR body and the deck structure, which gives rise to considerable cost savings.
The buoyancy member of the deck structure may comprise a barge-like construction that is provided with the coupling means near keel level. In an alternative embodiment, the buoyancy member comprises two spaced apart floating elements between which the connecting element of the upright SPAR body can be placed. After connection of the SPAR body and the deck structure, the buoyancy member may be raised above water level and can be used as a structural part of the deck structure for instance serving as personnel quarters. Alternatively the buoyancy member may be decoupled after completion of the SPAR buoy according to the present invention.
According to a specific embodiment of the method according to the present invention, the elongate body is in a horizontal position connected to the floating deck structure via a pivot construction. At the mooring site, the elongate body is placed in its upright position while pivoting it with respect to the deck structure around the pivot construction such that the coupling means of the deck structure are brought into contact with the coupling means of the elongate body. By the pivot connection, an accurate alignment between the deck structure and the horizontal SPAR buoy can take place. The pivot connection may be established at the mooring site, but can also be installed before combined transport of the deck structure and the elongated horizontal SPAR body, mutually connected by the pivot construction. During transport, additional reinforcing brackets may be mounted between the deck structure and the horizontal SPAR buoy for taking up the forces on the pivot construction and for providing a temporary increased stiffness between the deck structure and the horizontal SPAR body. At the mooring site the reinforcing brackets may be removed before erecting the SPAR body.
The SPAR body and/or the deck structure may be provided with horizontal thrusters that can propel the deck structure and the SPAR body during transport and which may after erecting of the SPAR buoy and connecting it to the deck structure, function to maintain the proper vertical position of the SPAR buoy.
Some embodiments of the SPAR construction method according to the present invention will by way of example be illustrated with reference to the accompanying drawings.